Just make a Superman movie, then a Batman movie, then a Wonder Woman movie and then tie them all together in the Justice League movie bringing back Green Lantern and introducing The Flash.
It's not even difficult.
Why should they copy Marvel?
It gives a chance to showcase the new versions of characters as well as ones who are just now getting a movie in an environment where they get plenty of screen time and characterization so they know who is popular enough to focus on in the team and who to give the shaft and to also allow the audience to give a crap about these guys before putting them in a team movie that has to divide up the screen time between the characters.
Seriously do you really think DC should risk screwing this up becuase the characters come off as weak in the big film, instead of using the solo films to figure out who works the best and who needs a recast?
By this line of reasoning they shouldn't have included Iron Man if Downey Jr. bombed bad with the box office and critics/fans?
I certainly didn't like the Thor movie at all for the most part but they still had him in the movie because he was part of the initial comic lineup.
The whole Marvel thing was a huge gamble.. even if the solo movies were successes the team movie could have ruined it all by being all flash and no substance. Marvel didn't have contingency plans because these movies need years to be done and were tightly integrated into the entire universe so even if an actor turned out bad he still would have been in the movie.
The exception of course if Norton's Hulk/Bruce Banner.. while being very good in his own movie they still recast him and it worked but i'd dare say it was not only because they found a good actor to replace him but the writing of Whedon that made the Hulk (an otherwise boring character to me) the breakout star of the movie.
IF WB sets a similar timetable like Marvel they have committed themselves like Marvel and i don't believe lightning can strike twice so i'm very hesitant to just copy Marvel's approach.
They tried the big budget, effects heavy summer blockbuster with Green Lantern and it didn't work because the elements of the movie didn't click like they did with Iron Man so DC might need to try a different approach and bringing in Nolan might be the first correct step.